1. Field of the Invention
The present invention relates to a device for generating entangled photon pairs. The device is applicable to quantum cryptographic systems, quantum computers, and other quantum information communication systems that exploit quantum correlation of photon pairs.
2. Description of the Related Art
In recent years, information technology has reached the quantum mechanical level. Quantum cryptography and quantum computing are attracting attention. In particular, quantum cryptography, in which the security of an encryption key is guaranteed by the principles of quantum mechanics, is now regarded as the ultimate secure cryptographic communication system and has been under active research and development.
A source of quantum correlated, that is, entangled photon pairs is an essential element for realizing advanced quantum information communication systems taking advantage of the quantum nonlocality of photon pairs.
One known method for generating quantum entangled photon pairs uses spontaneous parametric down-conversion (SPDC) in a second-order nonlinear optical medium.
In U.S. Pat. No. 7,211,812 (Japanese Patent Application Publication No. 2003-228091, now Japanese Patent No. 4098530), Takeuchi describes a quantum entangled photon pair generating device using β-BaB2O4 (BBO) crystals as second-order nonlinear optical media. Two BBO crystals are aligned in series with a half-wave plate centered between them. Input of linearly-polarized excitation light (pump light) with a wavelength of 351.1 nm produces spontaneous parametric down conversion in the BBO crystals, generating quantum correlated photon pairs with a wavelength equal to twice the wavelength of the excitation light (equal to 702.2 nm). The two photons in each pair are referred to as the signal photon and the idler photon. The half-wave plate rotates the polarization of the photons generated in the first BBO crystal by 90°. When the intensity of the excitation light is sufficiently weak and the probability of the occurrence of spontaneous parametric down conversion in both BBO crystals simultaneously is negligible, the device outputs a signal photon beam and a spatially separated idler photon beam in which each photon in each beam has an equal probability of having been generated in each of the two BBO crystals, and its polarization state is a superposition of two states with mutually orthogonal polarization planes. The signal and idler photons in each pair are said to be polarization entangled in that both give the same result when their polarization is measured in the same way.
Many other systems using similar structures to generate quantum entangled photon pairs with wavelengths in the 700 nm to 800 nm band have been reported. Generating entangled photon pairs with wavelengths in the 1550-nm band, which is the minimum absorption loss wavelength band of optical fibers, would be very useful in anticipation of long-haul quantum information communication systems.
In Japanese Patent Application Publication No. 2005-258232, Inoue describes a 1550-nm quantum entangled photon pair generating device using periodically poled lithium niobate (PPLN) waveguides as second-order nonlinear optical media. This device has a fiber loop structure incorporating two PPLN waveguides and a polarizing beam splitter (PBS). The two PPLN waveguides are displaced so that their optical axes are mutually orthogonal. A femtosecond excitation light pulse with a wavelength of 775 nm and 45° plane polarization is input through the PBS, which splits it into photons having equal probabilities of being aligned in polarization with the axis of each PPLN waveguide. Like the BBO crystals described above, the PPLN waveguides generate quantum correlated photon pairs by spontaneous parametric down conversion, but the signal and idler photons have wavelengths of 1550 nm.
A 1550-nm wavelength quantum entangled photon pair generating device using a PBS and a polarization maintaining optical fiber loop with a single PPLN element has been described by Lim et al. in Stable source for high quality telecom-band polarization-entangled photon pairs based on a single, pulse-pumped, short PPLN waveguide (Optic Express, vol. 16, No. 17, pp. 12460 to 12468, 2008). The polarization maintaining optical fiber loop also includes a fusion splice with a 90° twist. The PPLN waveguide generates quantum correlated photon pairs including signal photons with a wavelength of 1542 nm and idler photons with a wavelength of 1562 nm by spontaneous parametric down conversion. When the intensity of the excitation light is sufficiently weak, the state of each quantum correlated photon pair output from the PBS is a superposition of a state produced by clockwise travel around the loop and an orthogonally polarized state produced by counterclockwise travel.
The essential components of these known devices are a second-order nonlinear optical medium in which the SPDC process takes place, and a source of excitation light with a wavelength approximately half the wavelength of the desired quantum entangled photon pairs. In order to obtain quantum entangled photon pairs with wavelengths in the 1550 nm band for use in optical fiber communication, a 775-nm excitation light source is needed. This leads to the following problems.
The devices described by Inoue and Lim et al. require a PBS specially designed to operate similarly at both of two greatly differing wavelengths, e.g., 775 nm and 1550 nm. The lenses and other optical elements needed for internal and external optical coupling must also be specially designed. For example, the focal length of a lens must be selected to achieve optical coupling at both the 775-nm and 1550-nm wavelengths. Anti-reflection coatings that prevent reflection at both wavelengths are also needed. Thus the known art requires optical components capable of operating with excitation light and quantum correlated photon pairs having wavelengths that differ by a factor of two.
Generally speaking, a device having equally good performance characteristics for light with greatly differing wavelengths cannot be expected to have characteristics as good as a device optimized for one of the wavelengths. More specifically, the polarization extinction ratio of a PBS and the coupling efficiency of a lens system designed for operation at both 775 nm and 1550 nm are generally inferior to the polarization extinction ratio of a PBS and the coupling efficiency of a lens system optimized for just one of these wavelengths, e.g., 1550 nm. The use of components designed to operate at both wavelengths accordingly entails an excessive loss of both input excitation light and the quantum correlated photon pairs generated for output.
A quantum information communication system using quantum entangled photon pairs deals with extremely weak light to begin with, generating single photons or photon pairs or still smaller states and transmitting an average of one photon pair or less per time slot. A structure that leads to excessive loss of light critically impairs system performance, and calls for improvement.
It would be preferable for a system that generates entangled photon pairs in, for example, the 1550-nm band to use optical components designed just for operation in the 1550-nm band, including not only passive optical components such as the PBS and lens systems mentioned above but also active components such as light sources. Such optical components are commercially available at comparatively low prices and have proven high reliability, and the active components have excellent controllability.